Arsenic is found in many types of mineral deposits. Generally arsenic associates with iron, copper, nickel, cobalt, lead, antimony, silver and gold. This association is either as arsenic minerals or in solid solution with sulphide minerals. Copper, gold and lead mines are the primary producers of arsenic wastes. The amount of arsenic waste produced has increased steadily over the last two decades as the mining industry has been forced to begin exploiting complex ore bodies. This has been accompanied by a decrease in the market for arsenic, which is limited to the production of pesticides, lead acid storage batteries, glass, pharmaceuticals and wood preservation agents.
The complex ores containing arsenical minerals are usually sulphidic in nature. These ores require some form of oxidative treatment to achieve acceptable dissolution rates and metal liberation. It is usually during the oxidative process, such as bacterial oxidation or roasting, that the arsenic is released from its mineral form.
Safe disposal of arsenic-containing residues is an important consideration since many countries have strict mandatory limits on the allowable arsenic levels of wastes from processing operations. Arsenic-containing residues can be categorised as either solid waste, usually originating from a pyrometallurgical process, or aqueous wastes, originating from hydrometallurgical or hydrothermal processes, which are generally held in tailings dams.
The solid waste originating from pyrometallurgical oxidation is in the form of arsenic trioxide or in slag. In most cases arsenic minerals volatilise under high temperature to form arsenic trioxide (As203), which is collected in the flue dust treatment section of the plant. This powder is then stockpiled. The stockpiled material may be sold as is, purified, or treated to oxidise the arsenic to As(V) to form arsenic acid (H3As04).
In copper smelting a portion of the arsenic can be removed in the slag by blowing basic flux into the copper during the oxidation stage. Some of the arsenic also reports to the copper metal and the remainder to off-gases. The slag is dumped onto slag heaps. Off-gas cleaning by aqueous scrubbing typically produces acidic solutions rich in sulfur dioxide gas, iron in its ferrous state and arsenic in its arsenic (III) state. Neutralisation of this acid solution first requires oxidation of the ferrous iron and arsenic (III) in order to form stable precipitates for tailings disposal.
Pressure oxidation of ores containing arsenical minerals involves subjecting ores (usually gold ores) to high temperatures and pressures in an oxygen-rich atmosphere contained within an autoclave. During this process any arsenic which is solubilised will react with ferric iron to form crystalline ferric arsenate, (scorodite). Scorodite formation only requires a 1:1 ferric:arsenic (V) ratio, unlike amorphous ferric arsenate which requires a greater than 1:1 ferric:arsenic (V) ratio. This makes scorodite formation very attractive in ferric poor effluents. The technology exists to form scorodite from effluent streams in an autoclave operated at 150 C. The capital cost of an autoclave is, however, usually so high that scorodite precipitation may only be economically feasible if it is carried out together with the oxidation of the valuable concentrate. To the applicant's knowledge no process plants exist which employ an autoclave solely for formation of scorodite from effluent.
Hydrometallurgical treatment of arsenic-containing feeds results in aqueous effluent streams containing arsenic (III) and arsenic (V). These streams are generally treated by the addition of complexing agents such as Cd, Ca, Pb, Cu and Fe which react with the arsenic to form soluble compounds of varying solubility. The addition of calcium is usually in the form of lime or limestone to form calcium arsenate. Calcium arsenate is not environmentally stable as the compound reacts with carbon dioxide, releasing arsenic into ground water systems.
The addition of ferric iron to arsenic-containing feeds results in precipitation of arsenic (V) as a hydrated ferric arsenate and the arsenic (III) remains soluble in the aqueous effluent stream. The ferric arsenate precipitate is typically disposed of in tailings dams. The soluble arsenic (III) needs to be removed before the aqueous stream can be disposed of in an environmentally acceptable manner. Such additional process steps involve the use of additional reagents to oxidise the arsenic (III) in order to form stable precipitates for tailings disposal and can lead to significant increases in both capital and operating costs.
The effluent streams produced by the bacterial oxidation of copper concentrates contain arsenic in the form of arsenic (V), copper, ferric iron and ferrous iron. The ferric iron:arsenic (V) ratio is higher than 1:1.
The known prior art is typically concerned with pH adjusting the acid solution to a pH in the region of 7 to 10.
Kuyucak, N. (1998)(1) and U.S. Pat. Nos. 4,366,128 (Weir et. al.), 5,137,640 (Poncha), 5,427,691 (Kuyucak et. al.), 5,645,730 (Malachosky et. al.) and 5,651,895 (Gordon), all provide for disposal of arsenic in feed materials by adding a base to increase the pH. The pH adjustment of the effluent streams produced by the bacterial oxidation of copper concentrates to this level of pH would result in all the desired copper being co-precipitated with the arsenic.
The effluent streams produced by the bacterial oxidation of copper concentrates contain arsenic in the form of arsenic (V) and iron in the form of ferric iron. Oxidation of arsenic (III) and ferrous iron has already been achieved and hence oxidising agents do not need to be added to the effluent streams. Examples of such prior art are discussed hereinafter.
Zouboulis et. al. (1993)(2) and U.S. Pat. Nos. 4,241,039 (Koh et. al.), 5,024,769 (Gallup), 5,482,534 (Leonard et. al.) and 5,820,966 (Krause et. al.). This is typical of the prior art known to the applicant in that it relates to the oxidation of either ferrous iron to ferric iron and/or arsenic (III) to arsenic (V) prior to the neutralisation of arsenic.
Swash et. al. (1994)(3) and U.S. Pat. Nos. 4,149,880 (Prater), 4,244,735 (Reynolds et. al.) and 4,331,469 (Kunda), all provide for disposal of arsenic in feed materials as an iron arsenic compound. These processes however require pressure oxidation to operate in the sulphuric acid environment provided and are accordingly not cost effective due to the expensive nature of pressure oxidation.
Droppert et. al. (1995)(4), describes the production of crystalline scorodite from arsenic-rich sulphate-based metallurgical solutions at ambient pressure (95 C). Scorodite formation only requires a 1:1 ferric:arsenic (V) ratio, unlike amorphous ferric arsenate which requires a ferric:arsenic (V) ratio which is greater than 1:1. The effluent streams produced by the bacterial oxidation of copper concentrates have ferric iron:arsenic (V) ratios far in excess of 1:1. Droppert et. al. (1995)(4), does not describe copper co-precipitation or selective amorphous ferric arsenate precipitation.
Cashman, in U.S. Pat. No. 4,655,829, discloses a method of oxidising arsenic sulphide ores in a hydrometallurgical process with no soluble arsenic compounds formed. This is achieved by blending copper, lead or zinc sulphides to the arsenic sulphide ore. This approach is directed to the oxidative leaching of metal sulphides rather than the environmentally acceptable removal of arsenic from arsenic sulphide ores.